Small and thin imaging devices have come to be installed in personal digital assistants, which are small-sized and thin electronic equipment, such as a portable telephone and a PDA (Personal Digital Assistant). Whereby, it is possible to mutually transmit not only voice information but also image information to a remote place.
With respect to an image sensor used for such an imaging device, solid-state image sensing devices, such as a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal-Oxide Semiconductor) image sensor, are used. In order to reduce the cost, resin that can be mass-produced inexpensively has come to be used to manufacture a lens, which forms a subject image on these image sensors.
A type of optical system structured with three plastic lenses or with one glass lens and two plastic lenses is generally well known as an imaging lens used in the imaging device which is used in such a personal digital assistant. However, there is a technical limitation to achieving both of further microminiaturization of these optical systems and a mass-production capability required for use in a personal digital assistant.
In order to solve such a problem, a method of forming a lens and an optical system utilizing a replica method that simultaneously forms many lenses on one lens flat plate has been proposed (for example, refer to patent reference No. 1). Patent reference No. 1 discloses an imaging lens, in which a diffracting plane and a refracting interface are simultaneously produced on a lens flat plate and aberration is corrected.
However, it is not easy to form a diffracting surface and a refracting interface simultaneously on a lens flat plate. There are problems, in use of diffracting surface, that the diffraction efficiency decreases in wavelengths other than a design wavelength and that there is a large restriction to the angle of the incident light entering into the diffracting surface due to the poor angle characteristic of the incident light to the diffracting surface. Thus, there is a problem of difficulty in securing a wide field angle. In an optical system given in a patent reference No. 1, although the thickness of a lens element is not thick, there is a problem of a large astigmatism.
In the replica method, in order to easily give an aspheric surface shape, there is a technique in which UV hardening type resin is fixed by a metallic mold on a lens flat plate and UV hardening type resin is hardened by irradiation of a ultraviolet light. In this technique, in the case where the thickness of UV hardening type resin section is approximately greater than 0.25 (mm), there is a problem that a penetration of ultraviolet rays is poor and UV hardening type resin cannot be hardened sufficiently.
Patent reference No. 1: Laid-Open Japanese Patent Application Publication No. 2006-323365